1. Field of the Invention
This invention relates to a refractory composition for flow casting.
2. Description of the Prior Art
In recent years, there has been an increasing demand for monolithic refractories which are employed in the construction and repair of furnaces using methods such as flow casting, spraying or ramming. The flow casting method, simply called pouring, has gained increased importance because the method is very simple, and can be used for parts having complicated shapes and for those parts which cannot be constructed or repaired by spraying or ramming.
Refractories for flow casting should have sufficient flowability during the period required for casting, a hardenability which will produce sufficient strength to permit a rapid removal of the mold after casting (that is, shape retention at room temperature), and sufficient heat resistance at furnace temperatures. Such refractories must also have specified characteristics depending on the purpose of use.
Refractories of this kind have been called castable refractories, and those containing alumina cements, which in general having a composition comprising about 51 to 64% by weight Al.sub.2 O.sub.3, about 18 to 38% by weight CaO, about 6 to 8% by weight SiO.sub.2 and about 1 to 5% by weight Fe.sub.2 O.sub.3, as a binder have been widely used. Due to the use of alumina cement, these refractories have the following defects.
The strength achieved in alumina cement at room temperature is due to a hydration reaction, and the resulting hydration product contains about 10 molecules of water of crystallization (bonded water) per molecule of alumina cement.
Water added to castable refractories comprising alumina cement as a binder for achieving the flowability required for casting is partly consumed in the hydration reaction of the alumina cement and changes to water of crystallization in the hydrated compound to produce strength at room temperature. The excess water evaporates on drying after casting. The water remaining as water of crystallization in the hydrated compound escapes only very gradually even when heated to higher than 100.degree. C. Hence, when the temperature of the refractory after drying is increased to the furnace use temperature, the molded refractory sometimes explodes and is destroyed if a mistake in controlling the rate of temperature increase is made. Furthermore, since the strength of the hydrated compound decreases on dehydration of the hydrated compound, there is a temperature range within which a marked decrease in strength occurs before the temperature reaches about 1,000.degree. C. at which the sintering of the alumina cement due to heat reaction begins.
Alumina cement contains CaO and Al.sub.2 O.sub.3 as main components and the CaO content of the alumina cement can be as high as 18 to 38% by weight although this will differ depending on the type of alumina cement. Accordingly, even if the aggregate in the refractory containing an alumina cement as a binder is a high refractory material, since the binder portion of the refractory contains a large amount of CaO, marked chemical corrosion of the refractory material occurs when it comes into contact with molten metal or slag.
Use of phosphoric acid as a binder instead of the alumina cement, where after casting, the refractory is aged at 350.degree. C. to induce a reaction with the alumina aggregate is also known. However, this approach has the defect that because of the use of phosphoric acid, danger may be involved in its handling or in using the refractory.